On Friday, the FDA authorized the mRNA vaccine boosters for all those 18+ and a panel of CDC advisers voted in agreement. 

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The subject of who's eligible to receive COVID-19 booster shots in the U.S. has been confusing people for weeks.

Luckily, it just got a lot simpler: On Friday, the Food and Drug Administration authorized coronavirus booster shots of both the Pfizer-BioNTech and Moderna vaccines for adults 18+ in the U.S. who received their second shot at least six months ago. Reminder: Adults age 18 and up who received the Johnson and Johnson/Janssen COVID-19 vaccine at least two months ago were already eligible to get a booster shot as of late October, and certain populations — such as those age 65+ or at high risk of COVID-19 — were previously approved for Moderna and Pfizer boosters. It's also important to note that some states, including Arkansas, Colorado, and Maine, have already announced that all adults who want a booster shot can get it. (FYI, authorities also recently said it's ok to "mix and match" boosters.)

While undeniably exciting for many, the recent development doesn't technically mean that all adults can run to the pharmacy to get their COVID-19 vaccine booster shot right this second.

Here's the deal: Just hours after the FDA gave their authorization on Friday, the Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices (ACIP) gave their green light to expand booster eligibility. Now, however, CDC Director Rochelle Walensky, M.D. needs to sign off on it. Granted, she's expected to without an issue, but only then will everyone technically be able to get their booster shot, per federal health authorities.

The wait for this official expanded eligibility shouldn't be long, though. Typically, Dr. Walensky signs off on ACIP recommendations within 24 hours, says William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine.

The news comes just days after Anthony Fauci, M.D., director of the National Institute of Allergy and Infectious Diseases, said during a This Is Our Shot event on Wednesday that it was time to authorize boosters for all. "There's no doubt that immunity wanes," he said. "It wanes in everyone. It's more dangerous for the elderly but it is across all age groups." Dr. Fauci also expressed frustration that booster shots hadn't been authorized for all adult Americans. "Enough is enough," he said. "Let's get moving on here. We know what the data are."

Dr. Fauci specifically referenced data from Israel, which started giving boosters to adults over the age of 60 in late July. Then, in August, the country extended boosters to those age 40 and up, along with people considered high risk for COVID-19 complications like pregnant people and teachers. That data found a third dose of the Pfizer-BioNTech vaccine prevents infections in adults over the age of 60 by 11.3 times more than those who didn't get a booster. Boosters also lowered the rate of severe illness by 19.5 times.

Several studies have also shown that the effectiveness of the vaccines drops over time. Pfizer has released data noting that their third dose restored vaccine protection against COVID-19 to 95.6 percent (previous data showed the vaccine's efficacy falls to about 77 percent after 120 days, while another showed decreases down to 47 percent over the course of six months after the second shot). Moderna has said its booster dose increases COVID-19 antibodies up to 44 times that of non-boosted previous levels (data showed it was 93 percent effective at preventing COVID-19 after 5.3 months).

Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York, admits that the booster shot discussion has been confusing for many. "People are really confused about boosters," he says. "I get asked all the time who is eligible. With this, six months out if you've had the Moderna or Pfizer vaccine, and two months out if you've had Johnson & Johnson, all adults will be able to get one. That's much easier for people to understand."

Infectious disease expert Amesh A. Adalja, M.D., a senior scholar at the Johns Hopkins Center for Health Security, says that the FDA's move was "anticipated," adding, "it's happening now because there are many states and localities that are opening up boosters to the general public and the FDA does not want to lag their decisions and wants them to be congruent."

Dr. Russo says that opening up boosters to all is about more than just being easy to understand, though. "It's clear now that the vaccines' efficacy at preventing even mild infection is decreasing over time," he says. "And, in vulnerable individuals, a booster can help prevent more severe disease. By getting boosters, we will rectify this waning protection and get back to a level of protection we had with the first round of shots." Dr. Schaffner also notes that "there is some data to show that the boosters also reduce [the number of] minor infections" of COVID-19, making it less likely that you'll contract a minor breakthrough infection compared to if you only have two shots. (Related: What Is a Breakthrough COVID-19 Infection?)

Dr. Russo urges people who aren't concerned about getting a "mild" case of COVID-19 to consider this: "'Mild COVID' is defined as COVID that doesn't land you in the hospital. It can still be extraordinarily debilitating, and lingering symptoms can last for weeks and sometimes months."

That said, Dr. Adalja says he hopes the ACIP "has a robust discussion for where the benefit of boosters actually is and what the data is in healthy individuals." Meaning, the ACIP should specify the risk that vaccinated people have of actually getting sick, depending on how at-risk they are at baseline. That, he says, should help people decide for themselves if they actually need a booster. (Related: Half of COVID-19 Patients Experience Lingering Symptoms for Six Months, Says Study)

Doctors say it's difficult to tell what this will mean for COVID-19 vaccines going forward. "We shouldn't be surprised if we require a periodic booster the way we get our annual flu shot," says Dr. Schaffner. Dr. Russo, however, is hopeful that people may not need to get a COVID shot that often. "I think this is a three-shot vaccine at the end of the day," he says. "Unfortunately, we only got three to six months out of the first round, but I'm sure we'll get more time with the booster. Whether it will be a year or more, we just don't know yet."